The present invention relates to a semiconductor diode laser spectrometer arrangement and in particular an infrared semiconductor diode laser spectrometer having time resolved absorption, in which the wavenumber scale calibration is based on a time to wavenumber/cm−1 mapping.
Infrared absorption spectrometers are used for detecting and measuring gases. Infrared semiconductor diode lasers are used extensively to provide the light to be absorbed by the measurement species, as these lasers are relatively small, spectrally well defined, bright and tunable. Further advantages of these lasers over other lasers exist, some of which can be seen in spectroscopic monographs.
In remote locations and harsh environments, one of the most effective and accurate methods of trace gas sensing uses semiconductor diode laser based spectrometers. Although gas sensing has been undertaken for some decades, in many environments it remains difficult to remotely monitor trace gas constituents.
Many previous instruments have slow response times, are frequently bulky, unreliable, expensive, and require constant maintenance.
In order to retrieve information with known technology, remote sensing of gases usually takes place in the near and mid-infrared region of the electromagnetic spectrum, where the chemical fingerprints of most chemical compounds lie. By near and mid-infrared, it is meant radiation having a wavelength in the range of 1 μm to 14 μm. This spectral region contains highly transmitting windows, so-called “atmospheric windows”, which owe their transparency to the low density of strong absorption lines of CO2 and H2O. These atmospheric windows are of great interest for spectroscopy since the absorption lines of strongly absorbing trace molecules have similar or greater intensity than the weak lines of CO2 and H2O.
Near-infrared diode lasers produce light in the wavelength range of the vibrational overtones, about 1 μm to 3.0 μm. Since the absorption coefficients of the vibrational overtones are much smaller than those of the fundamental bands, the sensitivity of spectrometers that use such lasers remains limited. Thus, the sensitivity of such gas sensing apparatus rarely achieves the sub-part per billion (sub-ppb) range.
Mid-infrared diode lasers produce light in the wavelength range of the fundamental rotation-vibration bands, about 3 μm to 14 μm. These lasers have not been as technologically developed as those in the near infrared region, and hence have low single mode output power, Gas sensing systems based on mid-infra-red diodes are capable of achieving sub-ppb sensitivity. The development of such light sources has, therefore, been wholly dedicated to spectroscopic applications. Several disadvantages are associated with conventional mid-infrared diode lasers, principally lead salt lasers, such as low output power, and their need to be cryogenically cooled to 77 K or to even lower temperature. Thus, they require a bulky and expensive operating system to maintain this temperature.
Recently, room temperature and high light output power operation has been achieved in the mid-infrared using quantum cascade (QC) lasers. Unlike preceding lasers, QC lasers are unipolar semiconductor lasers that can be designed to emit at any desired wavelength in the mid-infrared. Replacement of lead salt lasers by QC lasers provides the potential to improve both the detection sensitivity and spectral resolution of mid-infrared absorption spectrometers.
The QC laser based spectrometers developed so far use two main approaches. The first uses a continuous wave (CW) operating QC laser as a “drop-in” replacement for a lead salt laser. The second approach is to use a pulsed QC laser in a way that mimics the use of a continuously operating laser. In some experiments conducted by Webster et al (Applied Optics LP 40, 321 (2001)), the first approach was used with one of the lead salt diode lasers in an ALIAS II spectrometer being replaced by a QC laser. Test measurements made using an ER2 aircraft platform showed that the QC laser could successfully replace a lead salt laser and was less affected by temperature instability. However, for CW operation the laser needed to be operated at 77 K. The second method was described originally by Whittaker et al (Optics Letters 23, 219 (1998)). In this method a very short current pulse is applied to a QC laser operating near room temperature to provide a narrow wavelength pulse. In this mode of operation the spectral resolution is limited by the wavelength up-chirp. Thus, in this type of spectrometer the wavelength up-chirp is regarded as detrimental to the operation of the system.
The wavelength up-chirp (“effective emission linewidth”) is induced by the temporal duration of the drive current/voltage pulse. By the term “effective emission linewidth”, it is meant the observable/measurable spectral width (FWHM) of the emission of a semiconductor diode laser induced by an applied current/voltage pulse to its electrical contacts.
For example, if the duration of the pulse applied to a QC laser were of the order of 10 ns, the effective emission linewidth would be of the order of 700 MHz (0.024 cm−1) in the spectral domain (Optics Letters 23, 219 (1998)).
In order to scan samples using a pulsed QC laser based spectrometer, the effective emission linewidth is runed across a spectral region using a slow DC current ramp superimposed on the pulse train. This means that the resultant spectral tuning is a quadratic function of the DC current ramp injected to the laser [Optics Letter 23, 219 (1998); Applied Optics 39 6866 (2000); Applied Optics 41, 573 (2002)]. A problem with this approach is, however, that an additional step is needed in the data processing stage, to correct for the quadratic effect. In some cases, to improve the signal to noise ratio, (Optics Letters 23, 219 (1998)), a small AC current modulation signal is added to the DC ramp in order to use phase sensitive detection of the detected optical signal.
Whilst adding this modulation may increase sensitivity, it requires the use of demodulation in the detection system, so rendering the system more complex. A further problem with this is that the use of a modulation inherently reduces the scan rate, since the high speed detected signals are demodulated to low audio frequencies signals. Hence, prior art arrangements of this type allow scan rates only of the order of tens of Hertz. One system proposed by Beyer et al (Third International Conference on Tunable Diode Laser Spectroscopy Jul. 8-12 2001, Zermatt Switzerland) uses the wavelength variation of the intrinsic wavelength chirp. However, the arrangement proposed is of limited use for chemical finger printing.
In both the CW operated laser (first method) described by Webster et al (Applied Optics LP 40, 321 (2001)) and the short pulse (second method), described originally by Whittaker et al (Optics Letters 23, 219 (1998)), for a gas with a small absorption coefficient the simplest way of achieving an observable change in the transmitted signal is to use a long sample length. This can be achieved by use of either resonant or non-resonant optical cells. Resonant cell schemes are complicated and require sophisticated techniques to minimise the effects of back-reflected signals from the input mirror to the cell disrupting the performance of the laser. Non-resonant cells, such as the so-called Herriot cell or astigmatic Herriot cell are attractive as they offer long path lengths, without the penalty of back-reflected signals. In addition, the path length is independent-of the concentration of the gas in the cell. A major drawback associated with non-resonant cells is the occurrence of “fringing” due to the partial overlap of the beams that propagate around the cell. This decreases significantly the system performance.
As can be seen, known spectrometers using semiconductor diode lasers, in particular quantum cascade (QC) lasers, have shortcomings, which limit their use for absorption spectroscopy in pulsed operation. Specifically prior art QC laser based spectrometers, where the light sources have to be driven in pulsed mode operation to achieve room temperature operation, have the resolution of their effective emission linewidth determined by the temporal duration of the drive voltage/current pulse applied to its electrical contacts.
An object of the present invention is to overcome at least one of the aforementioned problems.